Process for the electrolytic production of fluorine



April 1952 w. N. HOWELL El AL 2,592,144

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF FLUORINE Filed May 11, 1949 2SHEETS-SHEET 1 7 w w k 1 r Invpni'ors:

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Ap 1952 w. N. HOWELL ETAL 2,592,144

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF FLUORINE Filed May 11, 1949 2SHEETSSHEET 2 I nv eniors: VVjfZZiam N'armanJfi wpll g, v Jiarpld JiiZZ,3 3M, A i

l atented Apr. 8, i952 PROCESS FOR THE ELECTROLYTIC PRODUCTION OFFLUORINE William Norman Howell, Hale, Liverpool, and Harold Hill,Runcorn, England, assignors to Imperial Chemical Industries Limited, acorporation of Great Britain Application May-11, 1949, Serial No. 92,606In Great Britain May 14, 1948 This invention relates to improvements inelectrolytic apparatus and processes, and in particular, to apparatusand processes for the electrolytic production of fluorine.

Processes for the production of gaseous fluorine have been described inwhich a liquid comprising hydrogen fluoride and an alkali metal fluorideis subjected to electrolysis between a metal- 110 or carbon cathode andan anode which is more or less resistant to" fluorine and to theelectrolyte at the temperature of electrolysis. Materials which havebeen used as anodes in such processes include platinum, carbon andnickel. Hydrogen is evolved at the cathode and fluorine, with varyingamounts of oxygen and other impurities, dependent on impurities in theraw materials of the electrolyte, at the anode. Both anode and cathodegases contain greater or less amounts of hydrogen fluoride, depending onthe temperature at which electrolysis is carried out and on thecomposition of the electrolyte. Mixtures of hydrogen and fluorine giverise to violent explosions. It has therefore been regarded as essentialin fluorine cell construction to provide means for keeping separate thegases evolved at the cathode and the anode respectively. Various methodsof construction have been suggested for this purpose, as for example,the use of U-shaped or V-shaped cells having the electrodes in the sidelimbs and widely sepa-- rated from each other. In another method ofconstruction the upper portion of the cell is divided into cathode andanode compartments by means of a solid partition which may have adownward extension provided with slots or other apertures. Still anothermethod of preventing the mixing of the electrolytic gases employsimpervious partitions attached to the cover of the cell and dipping ashort distance into the electrolyte. Due to the corrosive nature of theelectrolyte and of the products of electrolysis, partitions require tobe of robust construction in order to ensure reasonable durability;their use generallyinvolves an anode-cathode separation of at leastseveral centimeters. Further, when partitions are employed, it may benecessary, in order to avoid troubles arising from bipolarlty of thepartition, to restrict the overall voltage of the cell. K A principalobject of this invention is the provision ofnew improvements in theprocess for the electrolytic production of fluorine and an apparatus forcarrying out such processes. ther object is the provision of a newmethod of Withdrawing fluorine generated during electrolyv 6 Claiins.(Cl. 204-60) A fur- 2 sis from the electrolytic cell and the provisionof new types of fluorine-producing cells which make possible this newtype of operation.

Another object is the provision of new types of such cells which have arelatively long period of life, i. e., in which the components of thecell are relatively slowly destroyed due to the corrosive action of theelectrolyte and the electrolysis products under the conditions ofelectrolysis. Still further objects and the entire scope ofapplicability of the present invention will become apparent from thedetailed description given hereinafter; it should be understood,however, that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the invention will become apparent to those skilledin the art from this detailed description.

These objects are accomplished according to the present invention by aprocess which comprises electrolysing a liquid mixture of the fluorideof a metal belonging to group 1A of the periodic table and having anatomic weight greater than 38 and less than 133, and hydrogen fluoride,and removing fluorine from the electrolysis zone through the material ofa gaspermeable, porous anode which is wholly im-' mersed in theelectrolyte and resistant to attack by the electrolyte and by fluorineunder the conditions of electrolysis. K A more complete understanding ofthe new procedures and apparatus involved with this invention can be hadby reference to the accompanying drawings in which: l r

Figure l is a diagrammatic'side view of a preferred form of cell for usein this invention in which the anode is constructed with a hollowinterior which is connected to the exterior of the cell by a suitableconduit and in which fluorine developed by the electrolysis is gatheredin the hollow interior, which is held entirely free of electrolyte, andwithdrawn through the conduit.

Figure 2 is an alternative form of cell for use invthe invention havingan anode which is not provided withany interior chamber and in whichfluorine is collected by passing through the pores of the gas-permeableanode and conveyed out of the cell through a conduit suitably infixedand located in the anode. It is here to be noted that the totalimmersion of the anode in the electrolyte is an essential feature of theinvention. If the anode is not so located, but obtrudes above the levelof the electrolyte surface, fluorine escapes from the non-submerged partof the anode, and only a portion of the gas passes through the providedconduit.

Referring in detail to the drawings, in Figure 1 the cell comprises amild steel container l for the electrolyte 3. A jacket 2 adapted forwater or steam heating surrounds the container I. The anode 4 is formedfrom a block of porous carbon wholly immersed in the electrolyte and hasa central chamber 5 into the upper end of which is inserted with aforced fit a tapered copper tube 5 so as to make a substantiallygas-tight joint when sealed by the molten electrolyte. The copper tubein addition to providing an exit for the fluorine from the chamber 5,serves the function of a support for the anode and of a means ofconducting the electric current thereto. Surrounding the anode andlocated a short distance therefrom is a cathode 1 of mild steel gauze,supported by a copper rod 8 which also serves as a conductor for theelectric current. In the drawing, the copper rod 8 is secured to aninsulatin pillar 9 bolted to the wall of the cell container, this rodand the tube 6 may, however, be supported in any convenient manner bymeans external to the cell.

Referring to Figure 2, in which parts similar to those shown in Figure 1are similarly numbered, the cell again comprises a mild steel containerl for the electrolyte 3, and a jacket 2, adapted for water or steamheating surrounds the container. The anode 4a is composed of a solidblock of porous carbon wholly immersed in the electrolyte and has acopper tube 6 infixed in its upper end. As long as the anode mass iswholly submerged in the electrolyte the fluorine, passing through thepores of the carbon mass 4a, leaves the said mass substantially entirelythrough the copper tube 6, which tube also serves 7 the function of asupport for the anode and of a means of conducting the electric currentthereto. The other features shown in Figure 2 are identical with thoseshown in Figure .1.

Procedures of this present invention are further illustrated by thefollowing examples of actual operations in accordance with theinvention.

Example 1 The container of the cell wasa'steam-jacketed mild steelvessel having the internal dimensions 10 x 5 and 7 The anode block wascut from porous carbon having a permeability of 17. The dimensions ofthe block were 3" deep x 1% wide x 2 thick, and a vertical cylindricalwell A" in diameter was drilled to a depth of 2 in the upper 1% x 2"face. A copper tube, which fitted tightly into the upper part of thewell, served as fluorine outlet and as the electrical connection to theanode, and Was clamped so as to hold the upper face of the anode about 2/2" below the surface of the electrolyte. The cathode, a piece of 10mesh mild steel gauze 12" x 1 /2 was bent to form an open-endedrectangular box having 3" sides 1 /2" deep. This was slipped over theanode and held so that the gauze was about /g" from the four verticalfaces of the anode.

Suificient molten electrolyte of the approximate composition KF+1.8 HFwas placed in the cell container to 'fill it to a depth of 6 when theelectrodes were in position. Impurities in the electrolyte comprisedsmall amounts of sulphur and silicon compounds and moisture.

When the anode and cathode supports were connected to the positive andnegative terminals respectively of a source of direct current it wasfound that at an applied potential of about 5 volts and upwards fluorinecould be collected from the copper tube at a pressure corresponding to3" to 4" head of water.

The following measurements were made when the cell voltage was '7 voltsand the temperature of the electrolyte 97 C.:

Current, 11.5 amps. Anode current density (based on surface area ofvertical faces), 0.5 amps/sq. in.

HF content of anode gas, 12%. Fluorine content of anode gas. afterremoving HF,

89.5-91.5% (the balance was mainly oxygen, arising from moisture presentin the electrolyte).

Current efliciency on fluorine. 75% of theory.

Example 2 Using the cell and electrode assembly of Example 1 with afresh batch of electrolyte analysing approximately KF+ 1.8 HF, at atemperature of 96 C., the following current-voltage relationships wereobserved while fluorine of good quality was issuing from the anodecompartment:

Current (amps) Voltage This condition may be realized by the use ofgraphite or amorphous carbon of high permeability as anode material inconjunction with a substantially anhydrous electrolyte composed ofhydrogen fluoride and potassium fluoride in a molar ratio less thanapproximately 2.5: 1. Preferably the molar ratio of hydrogen fluoride tothe alkali metal fluoride should be between 1.8:1 and 2:1.

The gas conduit through which the anode gases leave the cell may be atube of copper or other material which is resistant to corrosion by theelectrolyte and the cell gases, and which becomes anodically passiveunder the conditions of electrolysis. The tube may serve as theelectrical connection to the anode. It may, however, be ofnon-conducting material, in which case the current is conducted to theanode by separate and conventional means.

An electrolytic cell can be formed from a wide variety of materials andin any desirable shape. Obviously, the cell should be made of a materialwhich is not readily attacked by the electrolyte or products of reactionand a suitable material is mild steel. Cylindrical or rectangular shapesare suitable for the container which is preferably provided with ajacket for heating the container with hot'water or steam or with someother means for heating such as electrical immersion -heaters or thelike.

The anode may take a variety of shapes and is of necessity made fromsome material which is porous and gas-permeable but relativelyimpermeable to the electrolyte under the electrolysis conditions. Asindicated in connection with the drawings, the anode can be formedwithout any interior chamber in which case the generated fluorine isremoved through a suitably positioned conduit connected or fastened tothe anode. Both the anode and connecting conduits should be made from amaterial which is not readily attacked by the electrolyte or productgases during electrolysis. Suitable materials for the anode includeporous graphite or amorphous carbon and suitable materials for the gasexit pipe include copper. In one of the preferred forms of the cell, theanode is formed of a block of porous carbon having a centrally drilledchamber into which is press-fitted a tapered copper pipe so as to make asubstantially gas-tight connection when sealed with molten electrolyte.The gas exit tube, in

addition to forming an exit for the fluorine, can

also serve to support the anode and to make electrical connectionthereto.

The method which has been found suitable for forming the porous anode isto out it to the required shape in such a way that none of the originalouter surfaces of the block remain on the shaped anode. graphite is onehaving a permeability not less than 10, permeability as employed hereinbeing the number of cubic feet of air per square foot of surface capableof passing through a one inch thickness of the anode material per minuteagainst two inches water pressure.

The cell cathode can be formed in any desirable shape and from anysuitably inert material. Preferably the cathode is shaped so ascompletely to surround the anode at a spaced short distance therefrom.Suitable materials for making the cathode include mild steel gauze whichcan be supported by a copper rod to serve as a conductor for theelectrolysis current.

The electrolyte for use in the new process consists of a mixture ofhydrogen fluoride and the fluoride of an alkali metal of atomic weightbetween 38 and 133 which includes potassium, rubidium and caesiumfluorides. Mixtures of A suitable porous carbon or these alkali metalfluorides may also be employed.

Particular voltage and current densities employed using the proceduresand apparatus of this invention are not critical and the voltage mayvary, for example, between and volts and the current between 5 and 100amperes. Furthermore, the cell may be operated at subnormal orsuper-atmospheric pressures although it is preferable to operate thecell with a slight U pressure, e. g., a pressure equal to 1 to 5inchesof water, in order to insure that the interior chamber in theanode remains free of an electrolyte.

The temperature of electrolysis may be varied but should be sufficientlyhigh to keep the electrolyte in liquid condition and, preferably, shouldbe between 80 C. and 110 C. In operating the cell, the anode must bepositioned so as to be wholly immersed in the electrolyte.

The novel principles of this invention are broader than the specificembodiments recited above and rather than unduly extend this disclosureby atempting to list all the numerous modifications which have beenconceived and reduced to practice during the course of this development,these novel features are defined in the following claims:

1. A process for the production of fluorine which comprises flowing anelectric current through a liquid electrolyte mixture consistingessentially of hydrogen fluoride and a fluoride salt selected from thegroup consisting of potassium, rubidium and caesium fluorides andmixtures thereof, between a porous anode made of a material from thegroup consisting of graphite and amorphous carbon having a permeabilitynot less than 10 and an insoluble cathode, said anode being whollyimmersed in said electrolyte, and removing substantially all thefluorine generated by the electrolysis through the pores of the anodefrom the interior of the anode.

2. A process as claimed in claim 1, wherein said electrolyte is amixture of hydrogen fluoride and potassium fluoride with the molar ratioof HF to KF being between about 1.8 to 1 and 2.5 to 1.

3. A process as claimed in claim 1, wherein said cathode is made ofmetal gauze.

4. In a process for the production of fluorine,

the steps which comprise wholly immersing a porous chambered anode ofcarbon having a permeability not less than 10 in an electrolyteconsisting essentially of hydrogen fluoride and potassium fluoride inmolar ratio of between 1.8 to 1 and 2.5 to 1, passing an electriccurrent through said electrolyte between said anode and an insolublecathode and removing the fluorine from the electrolytic zone through theporous walls of said anode.

5. A process for the production of fluorine which comprises flowing anelectric current through a liquid electrolyte mixture consistingessentially of hydrogen fluoride and a fluorine salt from the groupconsisting of potassium, rubidium and caesium fluorides, and mixturesthereof, the molar ratio 'of HF to fluorine salt being between about 1.8to 1 and 2 to 1, between a hollow anode of porous carbon having apermeability of not less than 10 wholly immersed in said electrolyte andan insoluble cathode while maintaining the temperature of theelectrolyte between and 110 C., maintaining a pressure equal to 1 to 5inches of water within the hollow interior of said anode, and removingthe'fluorine from the electrolytic zone through the porous walls of saidanode.

6. A process as claimed in claim 5, wherein the electric voltage isbetween about 5 and 15 volts and the current between about 5 andamperes.

WILLIAM NORMAN HOWELL. HAROLD HILL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,000,815 Berl May 7, 19352,207,734 I-Ieise et a1 July 16, 1940 2,468,766 Low May 3, 1949 OTHERREFERENCES Ofiice of Technical Services publication P. B. 32.205, TheGeneration of Fluorine, June 15, 1943.

